Django relies on user input in some cases (e.g.
django.contrib.auth.views.login() and i18n)
to redirect the user to an “on success” URL. The security check for these
redirects (namely django.utils.http.is_safe_url()) considered some URLs
with basic authentication credentials “safe” when they shouldn’t be.

For example, a URL like http://mysite.example.com\@attacker.com would be
considered safe if the request’s host is http://mysite.example.com, but
redirecting to this URL sends the user to attacker.com.

Also, if a developer relies on is_safe_url() to provide safe redirect
targets and puts such a URL into a link, they could suffer from an XSS attack.

In each major version of Django since 1.6, the default number of iterations for
the PBKDF2PasswordHasher and its subclasses has increased. This improves
the security of the password as the speed of hardware increases, however, it
also creates a timing difference between a login request for a user with a
password encoded in an older number of iterations and login request for a
nonexistent user (which runs the default hasher’s default number of iterations
since Django 1.6).

This only affects users who haven’t logged in since the iterations were
increased. The first time a user logs in after an iterations increase, their
password is updated with the new iterations and there is no longer a timing
difference.

The new BasePasswordHasher.harden_runtime() method allows hashers to bridge
the runtime gap between the work factor (e.g. iterations) supplied in existing
encoded passwords and the default work factor of the hasher. This method
is implemented for PBKDF2PasswordHasher and BCryptPasswordHasher.
The number of rounds for the latter hasher hasn’t changed since Django 1.4, but
some projects may subclass it and increase the work factor as needed.

If you have different password hashes in your database (such as SHA1 hashes
from users who haven’t logged in since the default hasher switched to PBKDF2
in Django 1.4), the timing difference on a login request for these users may be
even greater and this fix doesn’t remedy that difference (or any difference
when changing hashers). You may be able to upgrade those hashes to prevent a timing attack for that case.